The invention relates generally to the production of methane-containing gases.
Methane-containing gases of sufficiently high methane content may, for instance, be used as exchange gases for, that is, instead of, natural gas. In recent times, numerous proposals for the production of natural gas exchange gas have become known. The starting materials which may be used include coke-oven gas and liquid, lowboiling hydrocarbons such as, for example, benzine. However, it is also possible to use solid fuels such as coal or coal dust as well as high-boiling hydrocarbons such as heavy oil or heavy fuel oil.
If the latter substances, that is, fuels, are used, then it is advantageous for these starting materials to be initially subjected to a partial oxidation (gasification). This may be accomplished using known processes such as, for instance, the Koppers-Totzek process, the Shell process or the Texaco process. Which of these processes is most favorably used in a given instance depends primarily upon the type and characteristics of the starting material to be gasified. Similarly, the composition of the resulting gas of partial oxidation is dependent upon the starting material.
Normally, the gases of partial oxidation are subjected to a desulfurization subsequent to the gasification during which the sulfur compounds contained in the gases are removed therefrom in accordance with known procedures.
Since the gases resulting from partial oxidation have a high carbon monoxide content which, in any event, exceeds 45 percent by volume, it has heretofore been the practice not to immediately subject the desulfurized gas of partial oxidation to a catalytic methane-forming or methanization reaction. Thus, it has been found that an immediate reaction or transformation of the carbon monoxide-rich gas results in the deposition of carbon on the conventional nickel-containing catalyst for the methanization already within a short period of time. The reason for this is that untransformed carbon monoxide splits into a carbon component and a carbon dioxide component in the presence of these catalysts, even at lower temperatures below 200.degree.C. Consequently, the practice until now has been to catalytically convert a portion of the carbon monoxide in the gas of partial oxidation subsequent to desulfurization. As is known, the conversion reaction proceeds according to the following relationship: EQU CO + H.sub.2 O .fwdarw. H.sub.2 + CO.sub.2 ( 1)
the carbon dioxide formed during this reaction is then removed from the process by means of procedures which are likewise conventional.
The most diverse proven possibilities and combinations exist for the desulfurization and conversion. For example, the conversion may be carried out prior to the desulfurization since there are presently available sulfur-resistant conversion catalysts as well as sulfursusceptible catalysts.
After the desulfurization and conversion, the gas of partial oxidation, which still has more or less of a high carbon monoxide content and which has been more or less freed of carbon dioxide, is, in accordance with the prior practice, subjected to a catalytic methanization. This methanization or methane-forming reaction proceeds predominantly in accordance with the following relationship: EQU CO + 3H.sub.2 .fwdarw. CH.sub.4 + H.sub.2 O (2)
concomitantly, methanization of carbon dioxide still remaining in the gas proceeds according to the following reaction: EQU CO.sub.2 + 4H.sub.2 .fwdarw. CH.sub.4 + 2H.sub.2 O (3)
the known procedures outlined above possess certain disadvantages, however. Thus, on the one hand, large quantities of water vapor are required for the conversion reaction since, aside from the water vapor utilized in the reaction itself, large quantities of water vapor for the equilibrium are also necessary. On the other hand, water vapor is produced during the subsequent methanization reaction and this water vapor is not put to use and must eventually be condensed out of the gas.
It has also become known, in the methanization of converted water gas, to add water vapor after the conversion to the gas which is to be transformed for the purpose of suppressing the deposition of carbon onto the methanization catalyst. Such an addition of water vapor, however, likewise gives rise to certain problems. Thus, in the first place, it is necessary to heat the water vapor to the reaction temperature for the methanization. Furthermore, the water vapor which remains untransformed must again be separated from the gas subsequent to the methanization.